1. Field of the Invention
The present invention relates to a recording material comprising a substrate and a ceramic layer applied to a surface of the substrate and also a light-sensitive layer, as well as to a process for producing a recording material.
2. Description of Related Art
To produce the lithographic printing plates used in lithographic processes, image areas and non-image areas are typically produced on a substrate, with the non-image areas generally being hydrophilic and the image areas generally being oleophilic. Accordingly, oil-based printing inks are generally repelled by the non-image areas after water has been applied to the substrate. Both the non-image areas and the image areas are produced by illumination of a light-sensitive recording layer on the surface of the substrate. The illumination results in differences in the solubility of the image areas and the non-image areas.
The preparation of the substrate prior to application of the light-sensitive layer not only has to ensure that the light-sensitive recording layer adheres firmly to the substrate, but it should also permit the soluble image material to be removed after development. Substrate materials for lithographic printing plates include, in general, aluminum and aluminum alloys which have a layer of aluminum oxide on their surface and a light-sensitive recording layer applied thereto. The aluminum oxide layer can be produced using an oxidation process which is generally carried out electrochemically. Prior to the oxidation, the surface of the aluminum substrate can be cleaned and this is followed by an etching process which provides the surface of the aluminum substrate with a textured layer, thus increasing the surface area of the substrate, which in turn determines the strength of bonding between substrate and the recording layer. The textured surface also helps to increase water retention.
Disadvantages of the known methods of preparing the substrates of lithographic printing plates include at least the following. For example, a large amount of electric energy is typically required for roughening and oxidizing the substrate surface. In addition, the roughening achieved by etching can generally only be carried out only relatively slowly. A further disadvantage is that the reprocessing of the waste products formed during the anodizing and during the roughening of the substrate is expensive.
To avoid these disadvantages, DE-C 25 04 545 (DE '545) proposes coating an aluminum substrate of a lithographic printing plate by using aluminum hydroxide formed in situ, with the coating comprising particulate material having a mean particle size of from 0.05 to 3000 .mu.m and the particulate material being applied to the substrate prior to the in-situ formation of the aluminum hydroxide. The particulate material can be, for example, selected from the group consisting of titanium dioxide, zinc oxide, .gamma.-iron(III) oxide, barium titanate, aluminum oxide, cerium oxide and zirconium oxide. The particulate material is bound to the surface of the aluminum substrate by aluminum hydroxide formed in situ. The aluminum hydroxide is formed by exposing the aluminum surface coated with the particulate material to an oxidizing environment comprising water. Thus, the surface of the aluminum substrate is oxidized to form hydrated aluminum oxide which grows in the form of crystallites around the particular material to form a matrix which binds the particulate material firmly to the surface of the aluminum substrate. The particulate material can, for example, be dusted onto the surface of the aluminum substrate and subsequently be exposed to an oxidizing atmosphere. Likewise, the particulate material can be applied to the aluminum substrate from a dispersion of the particulate material in a liquid carrier, after which the major part of the liquid carrier or all of the liquid carrier is evaporated. Suitable liquid carriers include water, lower aliphatic alcohols such as methanol, ethanol, isopropanol, n-propanol, n-butanol and isobutanol, lower aliphatic ketones, aliphatic hydrocarbons having from about 6 to 12 carbon atoms, aromatic hydrocarbons and mixtures of these carriers.
DE '545 also discloses increasing the effectiveness of the binding process by additions of, inter alia, sodium hydroxide, sodium bicarbonate, sodium acetate, magnesium oxide, calcium oxide, calcium carbonate, barium carbonate, magnesium nitrate, calcium nitrate, calcium fluoride, barium nitrate and calcium acetate. To produce the coating, the coated aluminum substrate is generally first wetted with water and subsequently placed in an open vessel into which steam under pressure is introduced. The aluminum substrate is exposed to the steam at 100.degree. C., for example, for 15 minutes, and is then dried.
WO 94/05507 discloses a process in which particles whose particle size extends from 2 .mu.m to 15 .mu.m are applied to the surface of a substrate by a thermal spraying technique or by plasma spraying. The material to be applied is, for example, aluminum oxide (Al.sub.2 O.sub.3). The thermal spraying technique is based on flame spraying. Particular preference is given to a process using a plasma spraying technique in which the powder is sprayed on in an atmosphere of inert gas, for example hydrogen, nitrogen or argon or mixtures of these or other gases. The gas is heated by an electric arc, for example, to at least 10.sup.4.degree. C. (10,000.degree. C.), in particular to 2.times.10.sup.4.degree. C. (20,000.degree.). As a result, the energy consumption of this technique is very high. The same applies to flame spraying in which the support material is in close contact with a block of material which has a high thermal mass and is accordingly held at a prescribed temperature.
EP-B-0 087 469 describes a process in which a ceramic layer is formed on an aluminum substrate by applying a slurry of at least one monobasic phosphate and nonmetallic inorganic particles to the surface of the aluminum substrate and forming a ceramic coating on the aluminum substrate by firing the slurry at a temperature of at least 230.degree. C. The ceramic layer is then coated with a light-sensitive lithographic layer. Some of the particles in the slurry are metal oxide particles having average particle sizes of from 0.001 to 45 .mu.m, where the metal oxide particles are aluminum oxide particles. The ceramic layer comprises a reaction product of aluminum oxide with a monobasic phosphate and a reaction product of a metal oxide which is not aluminum oxide with a monobasic phosphate. The orthophosphate of the metal oxide is insoluble in an aqueous solution having a pH of from 6 to 12.